1. Field of the Invention
The present invention relates to a manufacturing method of a semiconductor substrate. In particular, the present invention relates to a manufacturing method of a semiconductor substrate obtained by bonding a single crystalline semiconductor layer or a polycrystalline semiconductor layer to a substrate having an insulating surface, such as glass.
2. Description of the Related Art
Integrated circuits have been developed, which use a semiconductor substrate called a silicon-on-insulator (SOI) substrate that has a thin single crystal semiconductor layer over an insulating surface, instead of a silicon wafer that is manufactured by thinly slicing an ingot of single crystal semiconductor. An integrated circuit using an SOI substrate has been attracting attention because parasitic capacitance between drains of the transistors and the substrate is reduced and a semiconductor integrated circuit is made to have higher performance.
As a method for manufacturing SOI substrates, a hydrogen ion implantation separation method is known (for example, see Reference 1: U.S. Pat. No. 6,372,609). The hydrogen ion implantation separation method is a method in which hydrogen ions are implanted into a silicon wafer to form a microbubble layer in the silicon wafer at a predetermined depth from a surface thereof, the microbubble layer is used as a cleavage plane, and a thin silicon layer (SOI layer) is bonded to another silicon wafer. In addition to thermal treatment for separation of an SOI layer, it is necessary to perform thermal treatment in an oxidizing atmosphere to form an oxide film on the SOI layer, remove the oxide film, and perform thermal treatment at 1000 to 1300° C. in a reducing atmosphere to increase the bond strength.
As one of the known examples of SOI substrates in which SOI layers are formed over glass substrates, a structure is known in which a thin film of single crystal silicon having a covering film is formed over a glass substrate having a coating film by a hydrogen ion implantation separation method (see Reference 2: Japanese Published Patent Application No. 2004-134675). Also in this case, hydrogen ions are implanted into a piece of single crystal silicon to form a hydrogen implanted layer in the piece of single crystal silicon at a predetermined depth from a surface thereof, a glass substrate is bonded to the piece of single crystal silicon, then, the piece of silicon is separated using the hydrogen implanted layer, and a thin silicon layer (SOI layer) is formed over the glass substrate.
These techniques are suitable for the case when flat surfaces are bonded to each other; however, if steps due to plural islands formed on the surface of a single crystal silicon are present, a substrate opposite to the single crystal silicon may warp in bonding and thus the contact area becomes small, and as a result, an SOI layer having a desired shape cannot be obtained in some cases. For example, when a glass substrate having a thickness of 700 μm is used as the opposite substrate and the interval between plural islands is 200 μm or more, the substrate may warp.